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Topics of discussion

Powder Metallurgy

Principle, process, applications, advantages

and disadvantages of powder metallurgy,

Processes of powder making and mechanisms

of sintering.

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Introduction

Powder metallurgy is the technology of producing usefulcomponents shaped from metal powders by pressing andsimultaneous or subsequent heating to produce a coherentmass.

The heating operation is usually performed in a controlled-atmosphere furnace and is referred to as sintering.

The sintering temperature must be kept below the meltingpoint of the powder material or the melting point of themajor constituent if a mixture of metal powders is used.

Therefore, sintering involves a solid-state diffusion processthat allows the compacted powder particles to bondtogether without going through the molten state. This, infact, is the fundamental principle of powder metallurgy.

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Why powder metallurgy?

When melting metal is undesirable or uneconomical. Fusion is not suitable when it is required to produce

parts with controlled, unique structures, such as porousbearings, filters, metallic frictional materials, andcemented carbides.

Complicated shapes are made more economically andconveniently than other known manufacturingprocesses.

Powder-consolidation technique is becoming more and

more economical because it eliminates the need forfurther machining operations, offers more efficientutilization of materials, and allows components to beproduced in massive numbers with good surface finishand close tolerances.


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Outline of major powder metallurgical

processing steps


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Connecting rod for BMW V-8

automobile engine fabricated from

steel powder, followed by forging.


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Selection of products manufactured by

powder metallurgical processes


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Metal Powders

Reduction of metal oxides.

Atomization of molten metals.

Electrolytic deposition. Thermal decomposition of carbonyls

Condensation of metal vapor.

Mechanical processing of solid metals.


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Reduction of metal oxides

Raw material is usually an oxide that is subjectedto a sequence of concentration and purificationoperations before it is reduced.

Carbon, carbon monoxide, and hydrogen areused as reducing agents. Following is thechemical formula indicating the reaction betweencarbon and iron oxide:

Metal powders produced by this method includeiron, cobalt, nickel, tungsten, and molybdenum.


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Atomization

Atomization is frequently used for producingpowders from low-meltingpoint metals suchas tin, lead, zinc, aluminum, and cadmium.

The process involves forcing a molten metalthrough a small orifice to yield a stream that isdisintegrated by a jet of high-pressure fluid.

When compressed gas is used as theatomizing medium, the resulting powderparticles willbe spherical.


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Production of metal powders by

atomization


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Properties of Metal Powders

The particular method used for producing a

metal powder controls its particle and bulk

properties, which, in turn, affect the

processing characteristics of that powder.


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Chemical composition

In order to determine the chemical

composition, conventional chemical analysis is

used in addition to some special tests that are

applicable only to metal powders, such asweight loss after reduction in a stream of

hydrogen, which is an indirect indication of

the amount of oxide present.


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Particle shape & Particle size

Particle shape. The particle shape is influencedby the method of powder production andsignificantly affects the apparent density of the

powder, its pressing properties, and its sinteringability.

Particle size. The flow properties and theapparent density of a metal powder are markedly

influenced by the particle size, which can bedirectly determined by measurement on amicroscope, by sieving, or by sedimentation tests.


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Specific surface.

Specific surface is the total surface area of theparticles per unit weight of powder, usuallyexpressed in square centimeters per gram(cm3/g).

The specific surface has a considerable influenceon the sintering process.

The higher the specific surface, the higher theactivity during sintering because the driving force

for bonding during the sintering operation is theexcess energy due to the large area (high specificsurface).


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Flowability

Flowability is the ease with which a powder will flowunder gravity through an orifice.

The flow properties are dependent mainly upon theparticle shape, particle size, and particle-size

distribution. They are also affected by the presence of lubricants

and moisture.

Good flow properties are required if high productionrates are to be achieved in pressing operations becausethe die is filled with powder flowing under gravity andbecause a shorter die-filling time necessitates a highpowder-flow rate.


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Compressibility and Compactibility.

Compressibility and compactibility are very

important terms that indicate and describe

the behavior of a metal powder when

compacted in a die.

Compressibility indicates the densification

ability of a powder, whereas compactibility is

the structural stability of the produced as-pressed compact at a given pressure.


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Sintering ability.

Sintering ability is the ability of the adjacent

surfaces of particles in an as-pressed compact

to bond together when heated during the

sintering operation.

Sintering ability is influenced mainly by the

specific surface of the powder used and is the

factor responsible for imparting strength tothe compact.


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Factors Affecting the Selection

of Metal Powders

1.Economic considerations

2. Purity demands

3. Desired physical, electrical, or magneticcharacteristics of the compact


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POWDER METALLURGY:

THE BASIC PROCESS

The conventional powder metallurgy process

normally consists of three operations:

powder blending and mixing.

powder pressing.

compact sintering.


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Blending and Mixing

Blending and mixing the powders properly is essentialfor uniformity of the finished product.

Desired particle-size distribution is obtained byblending in advance the types of powders used.

These can be either elemental powders, includingalloying powders to produce a homogeneous mixtureof ingredients, or prealloyed powders.

In both cases, dry lubricants are added to the blendingpowders before mixing.

The commonly used lubricants include zinc stearate,lithium stearate, calcium stearate, stearic acid,paraffin,acra wax, and molybdenum disulfide.


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The amount of lubricant added usually rangesbetween 0.5 and 1.0 percent of the metalpowder by weight.

The function of the lubricant is to minimizethe die wear, to reduce the friction that isinitiated between the die surface and powderparticles during the compaction operation,and, hence, to obtain more even densitydistribution throughout the compact.

Blending and Mixing


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Pressing and Compacting

Pressing consists of filling a die cavity with acontrolled amount of blended powder, applyingthe required pressure, and then ejecting the as-pressed compact, usually called the green

compact, by the lower punch. The pressing operation is usually performed at

room temperature, with pressures ranging from10 tons/in2 (138 MPa) to 60 tons/in2(828 MPa),

depending upon the material, the characteristicsof the powder used, and the density of thecompact to be achieved.


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The die cavity is designed to allow a powderfill about three times the volume (or height) ofthe green compact.

The ratio between the initial height of theloose powder fill and the final height of thegreen compact is called the compression ratioand can be determined from the followingequation:



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MECHANISM OF PACKING

Repacking or restacking to attain better packing densityit depends upon physical characteristics of the powderparticles.

Oxide films covering their surfaces gets rubbed off

This leads to metal-to-metal contact and, consequently,to cold-pressure welding

Further increase in pressure leads to interlocking andplastic deformation of the particles to take place

Plasticity of the metal-powder particles plays a majorrole during the second stage of the pressing operation.

As the compaction pressure increases, furtherdensification is increasingly retarded by work-hardening of the particle material and by friction.


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A typical plot between density and

compaction pressure


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Typical thermal cycle during sintering.


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The widely used conventional machiningprocesses are not capable of producing somegeometries, or machining several of the very

hard high-strength temperature-resistantmetals.

Moreover, an inherent byproduct ofconventional machining processes are chips orswarf, which are quite difficult to recycle andtherefore of low, value.


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Abrasive waterjet machining uses the erosion action of ahigh velocity water stream, containing abrasive particulatefor cutting and drilling applications.

Typical abrasives used include garnet, olivine sand andsometimes silca sand.

These abrasives are mixed with a waterjet at a pressure ofbetween 200MPa and 400 MPa.

The stream is focused and impinges on the workpiecesurface at velocities of up to 915 m/s.

Many metals have been cut using abrasive waterjets,including aluminium, steel, tungsten carbide and titanium.

The major disadvantages are high capital cost and highnoise levels during operation.


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During electrochemical machining the workpiecematerial is removed by making it the anode of anelectrolytic cell, causing dissolution of theworkpiece.

An electrolyte is pumped through a gap betweenthe tool and workpiece.

By appropriately shaping the tool, complex

surfaces can be produced. The process can achieve metal removal rates of

around 2 cm3/min for many common metals.


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During electrical discharge machining (EDM) a

shaped electrode of graphite or copper is

placed a small distance away from the

workpiece, and an electrical potential appliedbetween the electrode and workpiece which is

immersed in a dielectric, typically of light oil.

The general arrangement of the EDM processis illustrated in


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A potential of between 40 and 400 V (d.c.) and currentof up to 400 A is applied in pulses of between 180 and300 kHz.

As the pulse potential rises the temperature of the

dielectric between the electrode and workpieceincreases, causing a small volume of dielectric tovaporize, ionize and form a spark.

The high temperature spark melts and vaporizes a

small portion of the workpiece. Between each pulsefresh dielectric flows into the electrode-workpiece gap,flushing the metal removed from the workpiece.


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This machining method involves emitting electrons from a filament

and accelerating the electrons to high velocity (about 60% of thespeed of light), by passing them through an electrical potential ofbetween 100 and 150 kV.

Magnetic coils are used to focus and position the beam of electronsto a high power density of at least 106 W/mm2.

When the electron beam strikes the workpiece its kinetic energy isconverted into heat. The high power densities are sufficient toinstantly melt and vaporize any material, regardless of melting pointor thermal properties.

This allows holes to be drilled at penetration rates that far exceedrates achievable by any other process.

The major disadvantage of electron beam machining is that thewhole process must be carried out in a good vacuum (maximumpressure 1 Pa) and this contributes to relatively high equipmentcosts.


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Lasers can be used to cut, drill, weld, mark, heat treatsurfaces or apply clad layers.

When focused, the laser power density can be increased to800 000 W/m2, enough for heating, melting, welding andcladding applications.

For cutting and drilling a focused laser of between 1.5 x 106

and 1.5 x 108 W/cm2 is required. At these power levelsmaterial is removed by sublimation. Laser machining hasthe advantages of a low heat input rate, it is easily adaptedto automatic controls and rapid processing speeds arepossible.

The disadvantages are a high capital cost, some hole taper,and highly reflective metals can be difficult to machine.


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Ultrasonic machining is particularly suitablefor machining hard, brittle materials becausethe machining tool does not come in contact

with the workpiece. They are separated by a liquid (vehicle) in

which abrasive grains are suspended. Equalvolumes of water and very fine grains of boron

oxide are mixed together to produce thedesired suspension.


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Ultrasonic energy applied to the tool results in high-frequency mechanical vibrations (20 to 30 kHz). Thesevibrations impart kinetic energy to the abrasive grains,which, in turn, impact the workpiece and abrade it.

The machining tool must be made of a tough ductilematerial such as copper, brass, or low-carbon steel sothat it will not be liable to fretting wear or abrasion, asis the case with the workpiece.

Ultrasonic machining is employed mainly in making

holes with irregular cross sections. Both through and blind holes can be produced by this

method.

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